The role of the anterior intraparietal sulcus in crossmodal processing of object features in humans: an rTMS study.

Investigations in macaques and humans have shown that the anterior intraparietal sulcus (IPS) has an important function in the integration of information from tactile and visual object manipulation. The goal of this study was to investigate the special functional role of the anterior IPS in visuo-tactile matching in humans. We used the "virtual-lesion" technique of repetitive transcranial magnetic stimulation (rTMS) to test the functional relevance of anterior IPS for visuo-tactile crossmodal matching. Two crossmodal (visual encoding and tactile recognition and vice versa) and two unimodal delayed matching-to-sample tests with geometrical patterns were performed by 12 healthy subjects. We determined error rates before and after focal low-frequency rTMS applied over the left anterior IPS, right anterior IPS and vertex. During the manipulation of objects with the right hand, rTMS over the left anterior IPS induced a significant deterioration for visual encoding and tactile recognition, but not for tactile encoding and visual recognition. For the visual and tactile unimodal conditions, no significant alterations in task performance were found. rTMS application over right IPS when manipulating objects with the left hand did not affect crossmodal task performance. In conclusion, we have demonstrated an essential functional role of the left anterior IPS for visuo-tactile matching when manipulating objects with the right hand. However, we found no clear evidence for left IPS involvement in tactile encoding and visual recognition. The differential effect of rTMS on tactile and visual encoding and recognition are not consistently explained by previous concepts of visuo-tactile integration.